Isolation bearings are used to add damping or increase a response period of a structure, such as a bridge. The five core performance functions of an isolation bearing are to transfer a vertical load, allow for large lateral displacements, produce a damping force, produce a spring restoring force, and allow for structure rotation. Two fundamental types of isolation bearings are used to accomplish these performance functions: sliding bearings and steel reinforced elastomeric bearings (SREB). Sliding bearings provide damping to a structure through frictional energy dissipation, but must include additional means to provide a restoring spring force. Elastomeric bearings provide restoring forces, but must include additional means to provide damping to the structure. Sliding isolators can incorporate springs to provide a restoring force. The isolation bearing disclosed in U.S. Pat. No. 5,491,937, for example, incorporates elastomeric compression springs. Upon displacement, both sliding and spring compression occurs, providing the necessary damping and restoring force requirements.
One drawback to sliding bearings with external springs is the space and cost required to fit the springs. Typically, compression springs can only be compressed to about 60% of their free length. At least one compression spring is required on each side of the bearing, meaning that the plan dimension of an isolator would be at least L=B+(2 d/0.6)=B+3.33 d, where L is the bearing plan dimension, B the load bearing element dimension, and d is the isolator seismic displacement. For small seismic displacements, this is typically not a severe limitation, but for large seismic displacements, the springs become overly-large and the bearing becomes too costly. In regions of high seismicity it is not uncommon to have seismic displacements of twelve inches and higher, resulting in bearing plan dimensions of forty-eight inches and larger. One problematic characteristic of such a bearing is that the spring rate is usually inversely proportional to spring length and proportional to its cross sectional area. Thus, if a long spring is used to accommodate a large seismic displacement, its diameter has to be large or the spring will be too weak. Thus, large seismic displacements cause both of the bearing's plan dimension and height to grow.
U.S. Pat. No. 4,599,834 describes a system in which a steel-reinforced elastomeric bearing's (SREB) upper surface is permitted to slide relative to the super structure, i.e., in essence sliding on top of an SREB. The center core of the SREB houses a friction element that is preloaded with compression springs, such that when the SREB displaces, sliding friction occurs. The internal friction mechanism serves to boost damping, as SREBs are typically low-damping bearings. Due to size constraints the mechanical spring friction mechanism is limited in the amount of vertical load it can support, e.g., it is not uncommon for bridge bearing loads to exceed 1,000 tons. Hence, for structural bearing applications the majority of the vertical load in such a design must be supported by the SREB. Further, displacement in the design is constrained to the central annular region. Since large displacements require large clearances, the practical design range is limited to small vertical loads and small displacements (e.g., mechanical equipment applications or small pedestrian bridges).
U.S. Pat. No. 5,867,951 describes a design in which a sliding isolator is stacked on top of an elastomeric bearing isolator. This approach prevents the isolator from sticking in one place due to static friction, thus allowing the isolator to attenuate high frequency vibrations. Shortcomings of this approach include the cost of profiling the sliding surface and the increase in structure elevation due to lateral displacement of the isolator.
Elastomeric isolation bearings can use both internal and external means to provide damping to the structure. A common external approach incorporates a central lead plug, to form a lead rubber bearing, such as described in U.S. Pat. Nos. 4,117,637, 4,499,694, and 4,593,502. Lead rubber bearing isolators are a widely-used type of seismic isolator. Elastomeric bearings in conjunction with dampers and mild steel elements have also been used, as described in U.S. Pat. No. 6,160,864. The elastomer can also be compounded to increase its damping capabilities, as in the case of high damping rubber bearings, as described in U.S. Pat. No. 6,107,389, but the level of damping is usually limited to less than 20% damping. Though rubber compounds exist with very high levels of damping, they exhibit high levels of creep, rendering them unsatisfactory for the vertical load performance function. A structure situated on a bearing with high creep properties would sag, leading to structural problems.
Restoring force issues aside, sliding bearings can be designed to accommodate high displacements by making the sliding surface larger. For SREBs, the problem is more complex. There are design limits on how much an elastomeric bearing can shear; if it displaces too much the isolator can buckle. One way to prevent this is to make the bearing larger in plan. But as the bearing grows in plan dimensions, it becomes stiffer in shear, and the height must be increased as well. Thus, the entire bearing grows. Another problem is that the axial compressive pressure decreases with increasing plan dimension; thus, lead rubber bearings require high pressures to help maintain lead core confinement.